It sounds easy: the Patent statute requires the naming of the actual inventors of the claimed subject matter. So if we just give credit where credit is due, why is this “one of the muddiest concepts” as noted recently by the Federal Circuit in In re Verhoef? A patentable “invention” begins with conception and ends with a reduction to practice. However, to be an inventor one needs to contribute to conception of at least one claim of the patent. Those who contribute only to the reduction to practice, such as by implementing the inventor’s specifications or just testing an invention, are not inventors. Different contributors sometimes have different or changing stories, and their contributions may become more or less important as the patent claims evolve. Consequently, it is often best to conduct an inventorship determination at filing – and write it down – keeping in mind that as the patent’s claims change during prosecution, the nature of the joint inventorship can be corrected for patent applications and issued patents.